Measurement of the ttH production cross-section in the H to bb decay channel

This PhD project is defined within the ATLAS collaboration, working on identifying Higgs bosons produced in association with a top-quark pair. This is one of the most complicated analyses performed by the ATLAS collaboration due the low signal and high background rates, coupled with the difficulties which come in associating uncertainties to a background process which is still being understood by the experimental and theoretical communities, namely top-quark pairs produced in association with heavy-flavour hadrons.

There will be opportunities to develop the next round of the ttH analysis based on Run-3 data. Run 3 will more than double the current dataset. The larger dataset allows increasingly differential measurements that are powerful for constraining potential Beyond Standard Model (BSM) effects through Higgs Effective Field Theories, while also challenging the theory predictions and requiring improved modelling to reduce the systematic limitations.

There will be scope to develop new machine learning methods as well as to incorporate ongoing ATLAS development that these analysis techniques and algorithms are based on. This could improve the performance of these algorithms as new b-tagging algorithms and jet objects are being developed.

The ttH process where the Higgs decays into b-quarks offers the highest statistics to probe the Higgs-top coupling. The analysis will exploit these events to probe the ttH process at high pT, where there is high sensitivity to BSM effects. The focus will be on two areas: first improvements to the selection of high pT events, leading to so-called boosted events, and secondly by incorporating improved modelling for the dominant top-quark pair background.

Working within the analysis team, the PhD student will improve the multivariate selection that is used to select boosted Higgs bosons. The student will also develop full reconstruction for the boosted ttH(bb) channel. This will allow differential measurements of variables involving the top-quark four-vectors that have increased sensitivity to a possible CP-violating Higgs-top coupling in the boosted regime.

The combination of these two work areas will result in differential measurements of ttH production from Run-2+3 data that will significantly improve on the Run-2 results and provide tight constraints on BSM contributions to the Higgs-top coupling. Potential beneficiaries include:

1. Particle physicists and others with a direct interest in fundamental particles, within and beyond the Standard Model. 2. Users of large-scale computing resources in many fields of science. 3. Physics graduates gaining from postgraduate teaching and projects as well as from PhD training within the group.

4. Firms employing our PhD graduates who will benefit from their leading-edge skills and ability to work in challenging international environments.